This invention relates to a multistage change-speed apparatus for a tractor. Tractors are used for quite a variety of works, and their running-travel-system change-speed apparatuses are therefore normally constructed to be able to change the speed in multiple steps more in number than in the case of usual passenger cars. Particularly remarkable is in slow speed area where such low range of 0.2-1.0 km/hour is employed, and most of them are made for shifting the forward speed in more than ten steps in total. Change-speed apparatuses of such construction inevitably require the transmission casing of large dimensions, together with complicated structure encased therein.
It has thus been a problem in what construction the multiple-step speed changing as extends to the super-low speed may be performed in a compact structure.
It has been the conventional practice to construct the change-speed apparatus for a tractor of the said type, as comprises a running-travel-system change-speed gearing apparatus, namely a main change-speed gearing just corresponding to the normal change-speed apparatus of the usual car, and an auxiliary change-speed gearing apparatus for shifting the output speed in high and low two steps. As a result of practically leasible designing of the entire change-speed apparatus including the meshing gears, however, it has been inevitable that the change-speed range as may be provided by the main change-speed gearing when the auxiliary change-speed gearing is shifted in high and the change-speed range as may be provided by the main change-speed gearing when the auxiliary change-speed gearing is shifted in low, partly overlap one with the other, thus it has not been able to make effective use of the multiple-step change-speed coverage.
Furthermore, it has recently been required for the tractors of the said type to utilize low speed range yet slower than the conventional so called super-low speed, but there has been a limit to the feasible super-low speed, because of the limited practical design possibility of the gears in the main and auxiliary change-speed gearings.
There have thus conventionally been no multistage change-speed apparatuses for tractors, as may realize multiple-step speed changing as extends to the super-low speed, sufficiently avoiding useless overlapping of the change-speed ranges as may be provided by the main change-speed gearing when the auxiliary change-speed gearing is shifted in high and low, and as may yet be compactly made up in the entirety of the change-speed apparatus of small dimensions.